MultipleMONOPTEROS-Dependent Pathways Are Involved in Leaf Initiation    

Abstract

Initiation of leaves at the flanks of the shoot apical meristem occurs at sites of auxin accumulation and pronounced expression of auxin-inducible PIN-FORMED1 (PIN) genes, suggesting a feedback loop to progressively focus auxin in concrete spots. Because PIN expression is regulated by auxin response factor activity, including MONOPTEROS (MP), it appeared possible that MP affects leaf formation as a positive regulator of PIN genes and auxin transport. Here, we analyze a novel, completely leafless phenotype arising from simultaneous interference with both auxin signaling and auxin transport. We show that mp pin1 double mutants, as well as mp mutants treated with auxin-efflux inhibitors, display synergistic abnormalities not seen in wild type regardless of how strongly auxin transport was reduced. The synergism of abnormalities indicates that the role of MP in shoot meristem organization is not limited to auxin transport regulation. In the mp mutant background, auxin transport inhibition completely abolishes leaf formation. Instead of forming leaves, the abnormal shoot meristems dramatically increase in size, harboring correspondingly enlarged expression domains of CLAVATA3 and SHOOTMERISTEMLESS, molecular markers for the central stem cell zone and the complete meristem, respectively. The observed synergism under conditions of auxin efflux inhibition was further supported by an unrestricted PIN1 expression in mp meristems, as compared to a partial restriction in wild-type meristems. Auxin transport-inhibited mp meristems also lacked detectable auxin maxima. We conclude that MP promotes the focusing of auxin and leaf initiation in part through pathways not affected by auxin efflux inhibitors.

Overcoming recalcitrance - Auxin response factor functions in plant regeneration

“…12,15 A gradient of MP expression, being low in the central zone and strongly expressed in the peripheral zone and particularly in lateral organ primordia, is consistent with MP's reported regulatory roles in the negative regulation of ARRs 15 in the central zone as well as in lateral organ emergence areas 7,16 in the peripheral zone. In reproductive shoot meristems this latter function has been shown to involve the LEAFY and AINTEGUMENTA LIKE/ PLETHORA transcription factors.…”

Regulation of preprocambial cell state acquisition by auxin signaling inArabidopsisleaves

“…To test this, we assessed the sensitivity to NPA of single and double mutant combinations of arf5-2 and athb8-11. Reduction in auxin transport frequently results in leaf fusion (Okada et al, 1991;Schuetz et al, 2008;Sieburth, 1999;Wang et al, 2005), a response that we first observed in wild type at 10 μM NPA (2/95) and in athb8-11 at 5 μM NPA (2/33). Approximately 10% (3/29) of arf5-2 seedlings displayed leaf fusion at 1 μM NPA, consistent with strong NPA hypersensitivity of mp mutants.…”

“…In double-mutant combinations of athb8-11 with arf5-2, no new phenotype class is observed; rather, the fraction of leaves displaying midvein bifurcation is increased to closely match that in the mp U55 allele. Strong mp/arf5 alleles display an exaggerated response to auxin transport interference that results in obstruction of leaf formation (Schuetz et al, 2008), whereas enhanced sensitivity 3243 RESEARCH ARTICLE ATHB8 preprocambial state acquisition to defective auxin flow in the weak mp allele arf5-2 most conspicuously manifests in leaf fusion at very low concentrations of auxin transport inhibitors. Under these conditions, leaves of athb8 mutants show normal sensitivity to auxin flow inhibition, but additional loss of ATHB8 function greatly increases the occurrence of leaf separation defects in auxin transport-inhibited arf5-2 seedlings.…”

“…Like athb8, mp seedlings display enhanced sensitivity to auxin transport inhibition (Schuetz et al, 2008). Because loss of ATHB8 function augments the effects of diminished MP activity on vein patterning, we asked whether the elevated response of athb8 to auxin transport inhibitors could be further exacerbated in a background of reduced MP function.…”

ERECTA Family Genes Regulate Auxin Transport in the Shoot Apical Meristem and Forming Leaf Primordia

“…In contrast, in the er erl1 erl2 mutant, the meristem is flatter and the leaf initiation rate is decreased. An increase in meristem size including an expansion of CLV3-expressing domains was also observed when auxin transport was inhibited in the background of mp, a mutant with reduced auxin signaling and decreased leaf initiation rate (Schuetz et al, 2008). Another common characteristic of SAM development in er erl1 erl2 and in auxin transport mutants is excessive cell elongation of L1 cells along the radial axis.…”